1. Field of the Invention
The present invention relates to a composite and a circuit board having a conductive layer on a resin layer and a method of manufacturing the same. More specifically, the present invention relates to improvements in a composite and a circuit board having a conductive layer formed through electroless plating on a resin layer and a method of manufacturing the same.
2. Description of the Prior Art
Conventionally known methods of manufacturing a composite having a conductive layer formed on a resin layer comprise a method of forming a metallic thin film on the surface of a resin layer utilizing a thin film forming technology such as evaporation, sputtering and the like, a method of forming a metallic layer on the surface of a resin layer by depositing catalyst nuclei for deposition of an electroless plated layer on the surface of the resin layer by immersing the same in an electroless plating bath, and the like. The conductive layer thus formed through such a thin film forming technology is as thick as several .mu.m at the largest. Accordingly, in using the conductive layer as a conductor of a circuit board, the impedance cannot be made small due to a small thickness thereof. In addition, generally a thin film forming process is disadvantageous in terms of productivity and cost as compared with other film forming processes, such as thick film forming process. Therefore, the thin film forming process has been merely utilized in the field where fine patterns are required, such as in manufacture of electronic devices such as circuit boards, magnetic heads and the like for use in industrial electronic machines.
Typical applications of formation of a conductive layer by electroless plating are plating of a conductive layer onto a through hole of a printed circuit board of a resin material, and the like. Other applications of electroless plating comprise a well-known full additive method for forming a circuit by forming a conductive layer of a thickness larger than 10 .mu.m by electroless plating only on a desired portion of a circuit board, and the like. In forming a conductive layer on a resin layer by electroless plating, the surface of the resin layer is processed to make uneven in order to enhance adhesion of the conductive layer onto the surface of the resin layer. Known methods for making the uneven (coarse) surface of the resin layers comprise a method of making the uneven surface in a mechanical manner, and a method of making the uneven surface through etching by chemicals, plasma and the like. However, the mechanical method involves less uniformity in an uneven state of the surface and the degree of unevenness is relatively large, with the result that the adhesion of a conductive layer to the surface of the resin layer is diversified particularly in case of a fine pattern of the conductive layer. On the other hand, the etching method by chemical or plasma involves a disadvantage that productivity is poor and the degree of unevenness is small.
An example of application of the conventional method of manufacturing a composite having a conductive layer on a resin layer is manufacture of a printed circuit board. Conventionally known methods of manufacturing a printed circuit board comprise a subtract method of forming a circuit by etching a copper foil laminated board, an additive method of forming only a desired circuit portion on a catalyst covered insulating board by a plating process, and the like. However, the former subtract method involves disadvantages that the amount of copper dissolved and removed by etching is much more than the amount of copper left to form a circuit, resulting in a waste of resource, and processes of forming an etching resist, etching and the like are complicated, with the result that difficulty is involved in uniformity of precision of the circuit pattern and realization of fine circuit patterns. On the other hand, the latter additive process comprises the step of forming a circuit pattern by plating a conductive layer only on a required circuit portion and is excellent from the standpoint of effective utilization of resource; however, the same also involves the difficulties that a specific resist having excellent durability to a plating solution is required since a plating process, usually an electroless plating process, is required for a long period of time, and the said resist layer need be formed on an insulating board in accordance with a circuit pattern by using a screen process, which is liable to cause inconvenience such as a bridge, an opened portion and the like of a printed pattern depending on the printing condition, thereby to make it difficult to realize a fine pattern. Another method is also proposed in which a concaved portion is formed on an insulating board using a pressing mold, whereupon a conductive paste is filled in the concave to form a circuit. Although this method is good in mass production of printed circuit boards of the same type, this method is not preferred in production of a less number of printed circuit boards having a number of types in that a number of pressing molds are required depending on the circuit patterns. In addition, in filling a conductive paste in the concaves, voids, bridges of the circuits and the like are likely to occur, so that deterioration of the electrical characteristic is unavoidable.